When an Administrator tries to disable "CAPTCHA on login" on the "Security Configuration" section, the change is not stored when Save button is clicked.
Sysadmin can edit the option but changes still won't work and CAPTCHA is still enabled and configured to 10 failed login attempts.

Option available at: /wiki/admin/viewsecurityconfig.action

To replicate follow:
Configuring Captcha for Failed Logins (Confluence Server documentation)

The option should be hidden as it is no longer managed by Confluence
There is already no documentation for this option on the Cloud space.

Notes:
Tested in JIRA + Confluence Cloud instance
Same configuration option is shown in JIRA and also doesn't work. JRA-43241